The application relates to an inverter, particularly to a phase-shift modulation resonant inverter.
Back-light inverter is widely used in notebook computers, instrumentation panels and other apparatuses in which LCD is employed. Under poor light or in total darkness, such inverter discharges a special AC voltage which makes a thin layer of film lighting at the back of the LC panel. The voltage frequency is several hundred Hertz (400 Hz-600 Hz) and the effective voltage ranges from several dozen to over one hundred volts. The back-light source is known as electroluminescent lighting (EL). Another type of back light source is fluorescent light with a working voltage frequency of 800-1,000 Hz and an amplitude of around 800 volts. The load of the above-mentioned back-light inverter is a small display panel and their output power ranges from 10 to several dozen volt-amperes. Simple free-running resonance circuit (see FIG. 1) is mostly employed and its efficiency is around 80%. Large pieces of electric vinyl film which came out in recent years calls for an inverter of much higher power for wider use in indoor lighting and large billboard lighting. A Canadian company by the name of Laservisions, for instance, came up with a luminous electric vinyl film as large as 2.2 m2 in 1999 which requires the back-light inverter to have an output power of over 1,200 volt-amperes and a maximum effective AC output voltage of 220V in order to achieve 700LUX luminance of the film. Only when there is a highly effective inverter large pieces of luminous material can be put into practical and extensive use.
A series of technical problems occur when the existing technology is employed to design a high-power inverter. Due to limited efficiency of free-running resonance circuit, circuit loss reach to several hundred watts when output power is higher than 1,000 volt-amperes. Moreover, as resonance induction power increases, noise is very high because resonance frequency of around 500 Hz is well within the sensitive range. In the case of high-power operation, it is very difficult to use conventional methods (e.g. vacuum dipping, mechanical compression or epoxy sealing of transformer) to degrade the noise caused by magnetostriction inherent in magnetic materials. An alternative circuit employs high-frequency pulse width modulation (PWM) technology to eliminate the noise caused by the magnetic materials. However, this method causes high loss of power at high-frequency switching, resulting in an efficiency below 80%, because the frequency of back-light inverter""s output voltage is 10 times that of AC power mains. Only when the switch modulation frequency is duly increased the distortion of sine-wave output voltage can be kept within tolerable range, resulting in higher switching loss. High heat loss and high temperature calls for strict ventilation and heat-sinking to ensure reliable performance of electronic components. Hence, high cost and sophisticated structure.
The object of invention is to provide a back-light inverter of high efficiency and low noise which is phase-shift modulation resonant inverter.
According to the invention, inverter comprises:
Switches Q1 and Q2 for turning on and off the inverter;
An inductor L and a load capacitor C for producing vibration output from the inverter;
A current sampling circuit for taking samples of resonance current;
A controller for adjusting the shift angle between the phase of switches Q1 and Q2 and that of resonance current.
This invention features simple structure, flexible control and a wide range of voltage changes, with an efficiency up to 98%.